1. Field of the Invention
This invention relates to a method of continuous casting a thin strip from a molten metal by a pair of cooling drums.
2. Description of the Related Art
A twin drum system (which uses a pair of cooling drums) is known as a method of continuous casting a thin strip having a shape approximate to a final shape in the art of continuous casting of metals. The outline of this system will be explained with reference to FIG. 1 of the accompanying drawings. A molten metal M is supplied from a tundish into a metal bath 4 defined by a pair of cooling drums 1, 1 and side walls or dams 3W, 3D, is then cooled by the cooling drums 1, 1 to form a solidified shell on the peripheral surface of each cooling drum 1 or 1. The solidified shells are pressed together and integrated at roll gap portions of the cooling drums 1, 1, and are delivered in the form of a thin strip S.
In the system described above, the side walls are necessary to hold the molten metal between the cooling drums, and have a structure such as the one described in Unexamined Utility Model Publication (Kokai) No. 63-90548, for example. As means for preventing the occurrence of leak of molten through gap between the side wall and the cooling drum end surface, there are known various methods such a the method that press the side wall onto the cooling drum end surfaces at a predetermined pressure or maintains a gap between them during casting, the method that moves the side walls in such a manner as to follow up the wear rate of the side walls (Unexamined Patent Publication (Kokai) No. 3-230848), the method that detects an open quantity of the side walls and regulates the press force (Unexamined Patent Publication (Kokai) No. 63-36954), the method that detects the opening force of the side walls and regulates the press force (Unexamined Patent Publication (Kokai) No. 53-177944), and so forth.
In accordance with the control system described above, which controls the press force to a constant level, however, it is very difficult to continue stable casting for long time. For, even when the press force is kept constant, both of the cooling drums and the side walls undergo non-uniform deformation due to the heat from pre-heating before casting and due to the heat from the molten metal during casting, and it is therefore difficult to get the conformable sliding surfaces of the cooling drum and side wall with each other at an early stage.
In other words, since the shape of the end surface of the cooling drum undergoes deformation due to the heat from the molten metal, the shape during casting is different from the shape before the casting. Accordingly, even if the sliding surface is made uniform in advance by pressing the side walls to the end surface of the cooling drums and rotating the cooling drums prior to the casting start, the resulting effect is not much better.
The side wall receives a reaction force from the cooling drum, such as a sliding friction, squeezing force of the mashy shell under the drum force of the cooling drum, and so forth. This reaction force is unstable. If this reaction force becomes greater than the press force of side wall during the press force constant control, the side walls are pressed back, so that it becomes difficult to keep the sliding motion uniform between the cooling drums and the side walls. In a case of press device using air cylinders, the retreat distance of the side wall when it is pressed back is so great that a gap is defined between the cooling drum end surfaces and the side walls, and causes leak.
On the other hand, it is not economical to maintain a large press force during the casting process because the wearing of the cooling drums and the side walls becomes considerably great. If a large friction groove or grooves occur on the side walls the edges of such grooves are easily to break and result in various casting problems such as entrapment of the broken pieces in the strip, a defective shape at the end portion of the strip due to the contact of the broken edge of the grooves with the solidified shell, and so forth.
Similarly, in accordance with the control system that keeps the gap constant, it is extremely difficult to keep the same throughout the full area of the sliding surface between the drum and the side wall during the casting period because non-uniform thermal deformation occurs in both the cooling drum and the side wall during the casting preparation period and at the initial stage of casting.
On the other hand, the method that moves the side walls in such a manner as to follow up the wear quantity of the side walls is easily to promote non-uniform wear and is not very practical. The method that detects the opening distance or the opening force of the side walls and regulates the press force is not suitable for practical casting operations because the separate measurement of the opening force and the press force is extremely difficult at the present stage and hence, it is extremely difficult to apply the press force in accordance with the detection quantity.